1. Field of the Invention
The present invention relates to a gap type discharge absorbing element to protect electric devices and circuits from overvoltage and in particular a conductive chip having an insulating gap is adopted as a spacer.
2. Description of the Related Art
Although a discharge tube glass-sealed with two opposed electrodes retained with a prescribed space therebetween is inexpensive to manufacture, the discharge lag and electrode wear are substantial. A gap type surge absorbing element has been considered, in which an insulating spacer of 50 to 100 .mu.m intervenes between opposed electrodes, a creeping discharge (primary discharge) is first initiated along the surface of the insulating gap, and next an arc discharge (secondary discharge) is induced between the opposed electrodes. The problem of discharge lag is not present with the two-stage discharge type. However, this type of surge absorbing element has not prevailed because of an imbalance of the discharge characteristics due to fluctuations and/or changes in materials and the shapes of the electrodes and thickness of the spacer. As a gap type surge absorbing element, the opposed electrodes are attached at both ends of a ceramic tube of which the outer circumferential surface is coated with a conductive plate, the middle of the plate is cut off by a laser beam to engrave micro gaps (50 to 100 .mu.m) and the conductive plate is divided into two (U.S. Pat. Nos. 4,317,155 and 4,727,350).
In a surge absorbing element in which micro gaps are formed by a laser beam, the opposed conductive plate edges having the micro gap therebetween appear with serrations when observed in enlargement. The serrations result in the discharge initiation voltage between the plates being unstable. Precision is required in technology for slicing a ceramic tube with a micro gap, thereby leading to high manufacturing costs. A surge absorbing element which is inexpensive in cost and has superior discharge characteristics has been strongly demanded in line with diffusion of electronic circuits in a wide range. The present invention has been developed in response to this demand.